This invention relates to gamma ray imaging and more particularly to an arrangement for providing enhanced images of bodies through improved processing of detected gamma radiation.
There are many techniques for irradiating bodies such as humans for diagnostic purposes. Typically, these usually involve the use of X radiation, gamma radiation, nuclear magnetic resonance imaging (MRI), or CT scanning. One such arrangement is described in U.S. Pat. No. 4,571,494 which employs an array of detector elements each comprised of semiconductor materials such as Si, Ge, CdTe, HgI.sub.2, or GaAs for detecting X or gamma radiation. The semiconductor detecting elements are arranged in an arc. The radiation is constrained to be incident upon the respective detector elements in their lengthwise directions by a collimator, e.g., a radiation absorbing body having holes or slits, interposed between the detector elements and the source of radiation. This orientation enables the detector elements to be essentially equally sensitive to the incoming radiation.
In arrangements such as the foregoing involving semiconductor detectors, the image information is individually read out. In others such as the Anger camera, e.g., see U.S. Pat. No. 4,672,207, the incident, collimated radiation is viewed by a large number of phototubes. The light generated by the conversion of an incident gamma ray into electrons in an NaI crystal is viewed by many of the photo tubes. Because of of the analog nature of the information, in past systems typical spatial resolutions of 5 to 6 mm FWHM are obtained. Resolution as good as about 3 mm FWHM has been reported.
Another problem with the foregoing arrangements is an apparent limit of 10% for energy resolution. This leads to background noise causing blurring of the image. When, as in nuclear medicine, gamma rays are emitted from a source within the body, such rays can undergo Compton scattering as they exit the body. Gamma ray interactions are well known and are discussed, for example, in "High Energy Particles" by Bruno Rossi, Prentice-Hall, 1952. These scattered gamma rays enter a collimator with less energy than their original value. With the available energy resolution capability of the NaI crystal in arrangements such as those referred to above, it is not possible to separate these lower energy scattered gammas from the full energy ones. Since the scattered gammas represent a larger number of photons than the desired ones and thus do not relate or point back to their original position in the body, resolution is limited.